The present invention relates generally to electro-optic modulators and, more particularly, to optical modulators having an internal structure for matching impedance with coaxial connectors.
The operation of electro-optic modulators is based on the interaction between an electrical microwave, or radio frequency (RF), modulating signal and an optical signal. An optical modulator is typically obtained by utilizing the electro-optical effect of the modulator""s waveguide material. This effect comprises changing, through an applied electric field, the index of refraction of the optical waveguide in which the optical signal propagates. This variation in time of the refractive index produces a desired phase modulation of the optical signal traveling through the waveguide. An amplitude modulator can be made by exploiting the above phase modulation in at least one arm of a waveguide interferometer, e.g., a Mach-Zehnder interferometer.
To obtain a modulator, it is necessary to have an optical waveguide carrying an optical signal and an electrode structure responsive to an applied RF signal that permits generation of the electric field necessary for modulating the optical signal. To increase the modulating effect, that is, the phase variation of the optical signal versus the amplitude of the applied RF signal, interaction between the optical signal and the electric field should be distributed along a planar microwave waveguide structure. The optical beam is made to propagate parallel to the planar microwave waveguide structure. In this way, the optical signal undergoes phase variations induced by the microwave signal along the entire length microwave waveguide.
An example of such a substrate and electrode structure is shown in FIG. 3. FIG. 3 is a top view of an electro-optic substrate 2 with an optical waveguide 1 running through it. Electrode 3 generates an electric field along its entire length. Thus, an optical signal propagating through waveguide 1 undergoes phase modulation along the entire length of electrode 3.
To obtain an increased modulating effect, proper electro-optic substrates, wherein the applied RF field can induce a significant variation of the refraction index, are exploited to guide the optical signal. An example of a useful material for such a substrate is Lithium Niobate, LiNbO3. A known, less preferred alternative material is Lithium Tantalate, LiTaO3.
Moreover, the coupling between the propagating optical and microwave signal must be synchronous to allow the phase variation induced by the microwave signal to increasingly add up throughout the whole structure. Synchronous coupling can be achieved by proper design of the microwave line, i.e. by making the effective index of the line equal to the optical effective index. This result may be obtained in several ways, for instance by increasing the electrode thickness and growing the electrodes on a thin, low dielectric constant, buffer layer.
The optimization of the electrode region with respect to synchronous propagation and maximum electro-optical interaction usually leads to lines of very small width, which cannot be directly connected to a planar-to-coaxial transition. This problem is usually solved by means of a transmission line taper, such as 5 in FIG. 3. The taper leads at a constant characteristic impedance from the small modulator line 3 to the comparatively large dimension at the exterior of modulator 2. Standard coaxial transitions require this larger dimension as an interface. The resulting input impedance levels of the modulator, however, tend to be much lower than 50 ohms, which is the standard reference impedance for which coaxial connectors and RF generators are currently designed.
This mismatch in source and load impedances results in numerous problems, such that the source and load impedances should be xe2x80x9cmatched.xe2x80x9d Impedance matching, as generally understood, comprises making a source impedance and a load impedance substantially equal, for instance, to allow the maximum transfer of electrical power from the source to the load. In the instant implementation, the source is an RF generator/coaxial cable, and the load is the optical modulator electrodes.
Input impedance matching is desirable in optical modulators, because besides increasing the input electrical power fed into the modulator, it also decreases multiple reflections and signal distortion. Because a change in refractive index in the substrate is directly related to the amount of RF electrical power input to the modulating electrode, the amplitude of optical modulation achievable at a given RF generator power is also increased when impedances are matched.
Patents in the field of electro-optic modulation describe various schemes, including providing an external matching network, for matching the impedance of optical modulators with their respective modulating signal sources.
U.S. Pat. No. 5,189,547 (Day et al.) describes a tunable adaptive external circuit connected to a bulk electro-optical modulator for impedance matching. This external driving circuit is connected between the signal generator and the modulator. The driving circuit includes discrete components that are hand-adjustable to match the impedance of the modulator with that of the signal generator.
U.S. Pat. No. 5,572,610 (Toyohara) describes an impedance matching means for matching an impedance of a control signal source and a signal electrode for a wide band waveguide-type optical device.
In the field of microwaves, a xe2x80x9cresonantxe2x80x9d line is a line connected to a load having a drastically different impedance from the characteristic impedance of the line itself. In electro-optical modulators, the characteristic impedance of the xe2x80x9chotxe2x80x9d (i.e., carrying the RF signal) electrode is typically several tens of ohms, for example 20-50 ohms. Typical configurations for a resonant modulator are: an open circuit RF electrode (xe2x80x9cinfinityxe2x80x9d impedance of the load); and the RF electrode short-circuited to ground (xe2x80x9czeroxe2x80x9d impedance of the load). Other configurations are possible, as a RF electrode connected to a load having an impedance of few ohms or of several kilo-ohms, for example. A good parameter which can be used in defining xe2x80x9cresonantxe2x80x9d is the modulus of the xcex93 coefficient, which is defined as:   Γ  =                    Z        L            -              Z        0                            Z        L            +              Z        0            
where: ZL is the impedance of the load, and
Z0 is the characteristic impedance of the line (RF electrode).
|xcex93| has a value in the range from 0 to 1. If |xcex93|=0, i.e., if ZL=Z0, the line is under a traveling-wave condition. If |xcex93|xcx9c1, i.e., if ZL=0 or ZL greater than  greater than Z0, the resonance condition is met. Henceforth, the following practical definition of resonance will be used: a modulator is of the xe2x80x9cresonantxe2x80x9d if type |xcex93|xe2x89xa70.5. A preferred resonance condition corresponds to |xcex93| greater than 0.8.
Resonant modulators are highly efficient in narrow bands around some resonance frequencies f0. Such high efficiency has been verified for frequencies around above some GHz, generally from 0.5 to 5 GHz, and preferably from 1 to 4 GHz. A typical frequency band of interest for resonant modulation is that around 2 GHz. An exemplary application of resonant modulators is phase modulation at 2 GHz for stimulated Brillouin scattering (SBS) suppression in cable television (CATV) systems. In such systems, high modulation efficiency can be exploited to save modulation power, resulting in less heating and reduced thermal stabilization problems.
For further details regarding resonant configuration and phase modulation for SBS suppression, please refer to WO 99/09451.
Impedance mismatch between the load and the line becomes a serious problem if the modulator design is of the resonant type. This problem gets worse the closer the |xcex93| value is to 1 near the resonant frequencies of interest. In this case, namely, the center band impedance becomes almost imaginary. Applicants have determined that the impedance of a resonant modulator, e.g. the modulator of FIG. 3, has typically a real (resistive) part lower than 10 ohms and an imaginary (reactive) part higher than 50 ohms. The impedance mismatch between the RF signal source and the modulator is conventionally eliminated by attaching to an external, concentrated network.
U.S. Pat. No. 4,372,643 (Liu et al.) discloses a standing-wave, velocity matched gate including an optical directional coupler that has a pair of electrodes located over the waveguide. The electrodes form an electrical transmission line that is energized at its input by a signal source having an output impedance R. In one embodiment, the transmission line is terminated by a short circuit and the electrodes are proportioned such that the input impedance of the line has a real part that is equal to R. The imaginary component of the transmission line is resonated by an external impedance connected across the input end of the line.
U.S. Pat. No. 4,850,667 (Djupsjxc3x6backa) relates to an electrode arrangement for optoelectronic devices. A first elongate electrode has a connecting conductor for an incoming microwave signal with the aid of which a light wave is to be modulated. The connecting conductor divides the first electrode into a standing wave guide and a traveling wave guide, which is connected via a resistor to a U-shaped second electrode. It is stated in the ""667 patent that the incoming modulating microwave has maximum modulating ability in the standing waveguide if its frequency is in agreement with the resonance frequency fc of the standing wave guide. In one embodiment, the connecting conductor is grounded.
U.S. Pat. No. 5,005,932 (Schaffner et al.) describes a traveling-wave electro-optical modulator with a periodic electrode structure of the intermittent interaction type. This electrode structure has a plurality of middle stubs to maintain the phase of the RF drive frequency in phase with the optical signal. This electrode structure makes possible the modulation of optical signals by RF signals above microwave frequencies. Impedance transforming and impedance matching characteristics are built into the modulator and this facilitates connection of the RF source since no extra impedance matching circuitry is required. Impedance transforming is performed by tapered input and output openings. The impedance matching is carried out by end stubs which are shorter than the middle stubs. The impedance matching stubs serve to transform the impedance of the periodic electrode structure to the impedance of an unperturbed linear RF coplanar waveguide. The impedance transforming sections serve to bring the impedance level seen by the RF signal at a location just outside the impedance matching stub up to the impedance level of the source and the load.
Applicants remark that impedance transforming circuitry and impedance matching features like those disclosed in the Schaffner et al. patent in combination with a traveling-wave relatively-broad-band electrode structure, cannot be used with a resonant modulator, as they would not allow to compensate the almost imaginary impedance of a resonant modulator.
Applicants have noticed that an external impedance matching network imposes additional costs, not only for the matching components, but also for the separate packaging and connectors for the interface between the modulator and the RF generator. Similarly, Applicants have recognized that the overall dimension of the system having an external impedance matching network is large due to these external components. As well, the reliability and repeatability of the external matching is undesirably low, due to variation in the component values and parasitic impedances of the external components.
Applicants have noticed that external matching, e.g., the simple integration within the package of a circuit comprising concentrated elements, such as capacitors, resistors and/or inductors, results in a very low reliability above 1-1.5 GHz. In such a case, the parasitic impedances induced by the strips used for soldering the concentrating elements in a circuit can cause a shift in the expected resonance frequency, resulting in an unpredictable impedance value for the integrated modulator and in an unreliable impedance matching between the RF signal source and the device.
Applicants have discovered that an appealing solution from the standpoint of space, cost, performance, and performance repeatability is provided by a matching network fully integrated within the optical modulator. Such an integrated matching network is realized, in the selected planar technology, on the same electro-optic substrate whereupon the modulator is implemented.
Applicants have further discovered that if the modulator is resonant, the matching network can be given a simple topology based on stub-line arrangements. In particular, the matching network may be placed between the modulator input and the coaxial connector, and may partially replace the constant impedance transition.
Applicants have still further discovered that the final taper leading from the matching section to the coaxial transition can be either with a non-optimal impedance or can be given an impedance so as to provide an additional degree of freedom in the matching section. It can be used as part of the matching network itself. By proper design and use of internally matched planar waveguides, an extremely compact layout may be obtained, thus enabling full compatibility, in terms of mounting, dimensions and position of external connectors, with previous designs based on external impedance matching.
In one aspect, an optical transmission system according to the invention comprises: an optical source for generating an optical signal; an RF signal source for generating an RF signal at a predetermined frequency, the RF signal source having an impedance; a resonant optical phase modulator for modulating the phase of the optical signal according to the RF signal; an optical amplifier for amplifying the optical signal to a power greater than 6 dBm; an optical fiber line for transmitting the amplified and phase modulated optical signal. The resonant optical phase modulator includes: an electro-optical substrate; an optical waveguide formed in the substrate and having a variable index of refraction; an active modulator electrode formed on the substrate in relation to the waveguide to effect electro-optical variation of the index of refraction upon application to the electrode of a modulating signal; an interface port formed on the substrate and providing the RF modulating signal to the electrode; an electrical structure, formed on the substrate and coupled to the interface port and the electrode, an impedance of the optical modulator including the interface port and the electrical structure being substantially equal to the impedance of the RF signal source.
In another aspect, a resonant optical modulator consistent with the invention includes an electro-optical substrate, an optical waveguide formed in the substrate and having a variable index of refraction, and a resonant active modulator electrode formed on the substrate in relation to the waveguide to effect electro-optical variation of the index of refraction upon application to the electrode of a modulating signal at a frequency around a resonant frequency. The modulator also includes an interface port formed on the substrate which provides the modulating signal to the electrode from a signal source, which has an impedance, and an electrical structure, formed on the substrate and coupled to the interface port and the electrode, for making an impedance of the optical modulator substantially equal to the impedance of the signal source.
Preferably, the active modulator electrode is connected to ground.
Preferably, the electrical structure includes a delay line connected between the interface port and the electrode, as well as a resonant stub connected at one end to the interface port. For example, the resonant stub is connected to ground at a second end or, in alternative, is open circuited at a second end. Typically the delay line has a length greater than xcex/40, where xcex is the wavelength in the delay line of a RF signal at the resonant frequency.
According to an embodiment, the interface port is tapered from the signal source down to its connection with the delay line.
Typically, the impedance of the signal source is 50 ohms.
Preferably, the resonant frequency is in the range of 0.5 to 5 GHz, more preferably in the range of 1 to 4 GHz.
In still another aspect, a resonant optical modulator consistent with the invention includes an electro-optical substrate, an optical waveguide formed in the substrate and having a variable index of refraction, and an active modulator electrode having a termination to ground and formed on the substrate in relation to the waveguide to effect electro-optical variation of the index of refraction upon application to the electrode of a modulating signal at a resonant frequency. The modulator of this aspect further includes an interface port formed on the substrate for providing the modulating signal to the electrode from a signal source, a first electrical element formed on the substrate and connected between the interface port and the electrode, and a second electrical element formed on the substrate and connected between the interface port and ground.
The total impedance of the electrode, the interface port, the first electrical element, and the second electrical element substantially equals an impedance of the signal source.
Typically an impedance of the electrode is mainly imaginary, and an impedance of at least one of the interface port, the first electrical element, and the second electrical element cancels the imaginary portion of the electrode impedance.
Typically the signal source includes a coaxial connector.
In a fourth aspect, a resonant optical modulator consistent with the invention includes means for modulating an optical signal in an electro-optical substrate, said means for modulating being formed on the substrate, means for providing an electrical modulating signal at a resonant frequency from a signal generating means, the means for providing being formed on the substrate, and means, coupled to the means for providing and the means for modulating, for causing an impedance of the optical modulator to be substantially equal to the impedance of the signal generating means. The means for causing is preferably formed on the substrate.
In an embodiment, the means for modulating is connected to ground.
The means for causing advantageously includes a means for delaying the modulating signal connected between the means for providing and the means for modulating and, preferably, the means for causing also includes a shunt means connected between the means for providing and ground.
Typically, the means for providing decreases in width from the signal generating means to its connection with the means for delaying.
Conventionally, the impedance of the signal generating means is 50 ohms.
In another aspect, an electrode structure consistent with the invention, for an optical modulator disposed on an electro-optical substrate having an optical waveguide extending through it, includes a first electrode symmetrically disposed between first and second portions of a ground plane, and having a width decreasing from an edge of the substrate to a node. A second electrode extends in one direction from the node and connects to the first portion of the ground plane. A third electrode extends in another direction from the node and has an end near the optical waveguide. A fourth electrode connected to the end of the third electrode near the optical waveguide, extends parallel to the optical waveguide and connects to the second portion of the ground plane.
Preferably an impedance of the electrode structure, viewed from an input to the first electrode, is substantially equal to an impedance of a signal source connected to the input of the first electrode. More preferably, a total impedance of the second, third, and fourth electrodes, viewed from the node, is substantially equal to the impedance of the signal source connected to the input of the first electrode.
In an embodiment, the third electrode includes at least two orthogonal portions.
Typically, a modulating signal in the fourth electrode modulates an optical signal traveling through the optical waveguide.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of the invention.